Humidity control thermostat and method for an air conditioning system

ABSTRACT

An air conditioner or heat pump system is equipped with a thermostat control which includes a temperature sensor and a humidity sensor and an algorithm to control the low voltage signals to the indoor blower section and the compressor to control operation of each in response to the indoor temperature and humidity conditions and the desired temperature and humidity setpoints.

BACKGROUND OF THE INVENTION

This invention relates to air conditioning and heat pump climate controlsystems and, particularly, to a method of operation control using athermostat equipped with a humidity sensor to control the humidity in anenclosed space.

In the design of an air conditioner or a heat pump climate controlsystem, it is general practice to use an indoor blower equipped tooperate at one of two or three possible speeds. As such, systemperformance and efficiency is optimized at a few sets of a much largernumber of possible sets of operating conditions. System performance atother conditions or blower speeds can be less than optimal but isconsidered acceptable on the basis of system design economy. Systemcontrol is accomplished using a thermostat to cycle the compressor andblower on and off in response to a demand for sensible cooling, thereby,maintaining the temperature inside the structure at a desired level.

The humidity control capability of an air conditioning system isexpressed as the ratio of sensible heat capacity to the total heatcapacity and referred to as the Sensible Heat Ratio (SHR). The SHR is afunction of the evaporator operating temperature, evaporator surfacearea, the dew point and the amount of the air entering the evaporator.One problem with typical system design is that the humidity controlcapability of the system is, in general, not optimized to the latentload requirements of the structure during all of its seasonal weatherconditions. That is to say, while the air conditioner or heat pump is insatisfactory control of the sensible structure load, the humidity levelis most often out of control. In fact, the indoor humidity levels duringsensible load conditions considered "light loading" greatly exceedlevels generally considered comfortable.

At outdoor dry bulb temperatures in the 65-85° F. range, airconditioning systems experience long idle times. That is, when theoutdoor temperature is near or below the indoor temperature setpoint,the air conditioner is off and no cooling or dehumidification isperformed. As at these outdoor temperatures, there is almost always aneed for dehumidification the indoor humidity may rise to unacceptablelevels resulting in mold, mildew and dust mite growth conditions. Inaddition, without dehumidification, the moisture content in the buildingstructural materials rises which may cause damage. If the humidity isallowed to raise excessively, condensation could form on interiorsurfaces and could cause furniture damage. The problem is therefore, todevelop a cooling system control that provides for dehumidificationoperation during idle periods which will keep humidity levels low enoughto discourage or eliminate the growth of mold, mildew and dust mites andto prevent damage to the building and furniture.

Many homes and structures remain unoccupied by humans for extendedperiods of time. These long unoccupied periods pose a special problem interms of dehumidification. Leaving the HVAC system to operate in orderto dehumidify the space while unoccupied needlessly over cools the spaceand is energy inefficient. If the HVAC system is not operated the spacewill encounter long periods when moisture will accumulate and willpromote the growth of mold and mildew. In addition, if the HVAC systemis not operated for long periods and moisture is permitted to accumulatethe HVAC system, once restarted, will require an extended period todehumidify the space. Occupancy sensing controls have been used inconjunction with HVAC systems in the past wherein the HVAC system iseither switched off or switched to a higher temperature setting. What isneeded is a device and control method for an HVAC system that providesfor an energy efficient amount of latent heat removal for use duringboth occupied and unoccupied periods.

Many attempts have been made to efficiently deal with thedehumidification of enclosed spaces. For instance many prior systemshave added separate dehumidification apparatus to conventional HVACsystems and are disclosed in U.S. Pat. Nos. 5,598,715, 5,578,753,5,427,175, 5,305,822, 5,231,845. U.S. Pat. No. 5,544,809 teaches amethod which utilizes algorithm timing strategies to optimize air dryingin an enclosed space. This method does not vary the SHR by controllingthe air conditioning system component operation. Instead the system in'809 enables the HVAC system at fill capacity for timed intervals andthen turns the HVAC system off for timed intervals based on temperatureand humidity measurements. The result of such a system is that, asoutlined above, the system may take a long time to actually catch up tothe latent heat load requirements and high indoor humidity and lowoutdoor dry bulb conditions. In addition, '809 teaches a method thatincludes an occupancy sensor that automatically extends the system cycletime if the enclosed space is unoccupied.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a device and method forcontrolling an air conditioning or heat pump system wherein indoorairflow and compressor operation are controlled during periods of lightsensible loading to increase the dehumidification capabilities and limithumidity buildup during these periods.

It is an object of a further aspect of this invention to provide adevice and method for controlling an air conditioning or heat pumpsystem wherein the use of a thermostat which is equipped with a humiditysensor input for direct measurement of the indoor humidity level and acontrol algorithm using the measured temperature and humidityinformation to modify the operation of the indoor blower and the airconditioner or heat pump to improve the humidity control in a structure.

Further, it is an object of an additional aspect of this invention toprovide an air conditioner or heat pump system which will respond to thethermostat signals in a manner which will result in improved humiditycontrol performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings form a part of the specification. Throughoutthe drawings, like reference numbers identify like elements.

FIG. 1 is a graphical representation of the temperature and humidity inan enclosed space employing an air conditioning system operated inaccordance with the first mode of the control method of the presentinvention;

FIG. 2 is a graphical representation of the temperature and humidity inan enclosed space employing an air conditioning system operated inaccordance with the second mode of the control method of the presentinvention;

FIG. 3 is a schematic representation of an air conditioning systemequipped with a particular embodiment of the control device of thepresent invention; and

FIG. 4 is a flow diagram illustrating the algorithm logic employed bythe microprocessor of the control device of the present inventionrepresented in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As will be described in greater detail below the present inventionprovides for governing an air conditioning system to control thehumidity in an enclosed space. It should be evident however to oneskilled in the art that the present invention is not limited to thespecific examples given and could be utilized in other systems wherelatent heat removal is required.

Referring to FIG. 1 there is illustrated a typical operational mode map1 for the present invention. The temperature 2 and humidity 3 setpoints,as well as the occupation mode setting (not shown), are selected by theuser. In the operational mode illustrated in this particular figure, theoccupation mode is set to "occupied". The temperature setpoint isdepicted graphically by the arrow 2 and could, by way of example,represent a typical setting of 77° F. Similarly the humidity setpoint isindicated graphically by arrow 3 and could by way of example represent asetting of 55% RH. These settings are typical user selections for anoccupied enclosed space.

The regions depicted in the mode map illustrated in FIG. 1 correspond todifferent operational modes of the HVAC system. In general, region Icorresponds to conditions when the temperature in the enclosed space isabove the setpoint temperature and the humidity in the enclosed space isbelow the humidity setpoint. Region II corresponds to conditions whenboth the temperature and the humidity in the enclosed space are abovethe respective setpoints. Similarly region III corresponds to conditionswhen the temperature in the enclosed space is below the setpointtemperature and the humidity in the enclosed space varies both above andbelow the humidity setpoint. Region IV corresponds to a discrete set ofconditions where the temperature of the enclosed space is below thetemperature setpoint and the humidity in the enclosed space is above thehumidity setpoint. These regions are normal operating regions for anycooling system and help to illustrate the operation of the presentinvention. In region I, a demand exists for sensible cooling only. Theair conditioning or heat pump system is commanded to operate to satisfythe sensible demand, but will also dehumidify to the extent limited byits SHR. This could lead to excessive drying of the indoor air resultingin a coolness feeling to an occupant. Region II is characterized bydemand for both cooling and dehumidification. In general, all airconditioners or heat pumps can meet the demand for cooling in thisregion, but fail to meet the demand for dehumidification when thehumidity exceeds the setpoint by more than a few percent. Attempts tomeet this demand using humidistats or other schemes to override thetemperature setpoint result in over-cooling, unless a second thermostat(or other device) is used to limit the over-cooling.

In accordance with the present invention, a single thermostat isutilized which monitors humidity as well as temperature to control theindoor blower and the compressor to improve the latent heat removal. Thepresent invention operates to extend the dehumidification performance ofthe system as needed beyond the temperature setpoint in region IV to asecond temperature limit as will be further defined below to improve thehumidity control of the system. In region I, the air conditioner or heatpump is controlled as a normal system where the air flow from the indoorblower is normal and the compressor and blower are cycled on and off asnecessary to meet sensible demand. In region II, the compressor isoperated normally, but the airflow delivered by the indoor blower isadjusted, in one embodiment to 70-80% of the normal airflow, to lowerthe evaporator coil operating temperature and increase the latent heatremoval capability of the evaporator, thereby decreasing SHR. If therelative humidity of the conditioned space is such that the temperaturesetpoint is satisfied before the air conditioner or heat pump hassatisfied the dehumidification demand, as depicted by region IV, thesystem will be allowed to continue to run to further dehumidify theenclosed space. In one embodiment of the present invention the operationof the HVAC system in region IV is subject to the following temperatureand humidity limits:

1. The humidity setpoint is satisfied and the temperature is above thelimits described in 2 and 3,

2. The humidity is more that 6% greater than the setpoint, but thetemperature is more than 3° F. below the temperature setpoint, and

3. The humidity is 0-6% greater than the setpoint and the temperaturehas fallen to more than 1° F. per each 2% RH error from the temperatureand humidity setpoints respectively.

During operation of the HVAC system in region IV, the indoor airflowwill be adjusted to as low a level as the system can operate withoutdamage to the compressor or any other system component due torefrigerant floodback or due to a freezing evaporator coil. In a typicalsystem normal airflow over the evaporator is 400 cubic feet of air perminute for every ton of cooling capacity (cfm/ton). In region IV thecompressor is also cycled on and off at a rate necessary to match loadrequirements but, not to exceed a maximum on period or duty cycle. Inone embodiment of the present invention, the compressor is run for a 10minute maximum on-time with maximum duty cycle of 50%. Both of the abovecontrol actions will maximize the latent heat removal capability of theair conditioner or heat pump while minimizing the sensible heatcapacity, thereby improving the humidity control characteristics of thesystem. By contrast, U.S. Pat. No. 4,003,729, which is commonlyassigned, teaches a method of dehumidification wherein the speed of afan is varied to decrease the amount of air flow across the evaporatorto maintain the refrigerant level at the evaporator at a predeterminedlevel. In the '729 patent there is a need to monitor refrigeranttemperature and the compressor cycle is not varied in conjunction withevaporator fan speed to optimize latent heat removal.

Referring to FIG. 2 there is illustrated a second operational mode mapfor the present invention where the occupation mode is set to"unoccupied". The temperature setpoint is depicted graphically by thearrow 2 and could, by way of example, represent a typical setting of 85°F. Similarly the humidity setpoint is indicated graphically by arrow 3and could by way of example represent a setting of 55% RH. Thesesettings are typical user selections for an unoccupied enclosed spacesuch as desired during a long period away from a home or building. Theoperation for all of the regions follow that of the occupied mode asshown in FIG. 1 and discussed above except for the following noted limitchanges.

In region II operations are identical to that described above exceptthat the temperature setpoint is usually selected by the user to behigher than that described above. In one embodiment of the presentinvention, the thermostat has a one button vacation feature which, whenselected, sets the temperature setpoint to 85° F. and the humiditysetting is maintained at the occupied mode setting.

In region IV the operation of the HVAC system is subject to thefollowing temperature and humidity limits:

1. The humidity setpoint is satisfied and the temperature is above thelimits described in 2 and 3,

2. The humidity is more that 6% greater than the setpoint but, thetemperature is less than 70° F., and

3. The humidity is 0-6% greater than the setpoint and the temperaturehas fallen to more than 1° F. per each 1% RH error from 76° F. and thehumidity setpoint respectively.

Another embodiment of the present invention anticipates the use of amultiple speed or variable speed compressor. In this embodiment thelowest compressor speed available is used in region IV to further limitthe sensible capacity and maximize the latent capacity of the system.

Referring to FIG. 3 there is depicted an HVAC climate control system 10incorporating the control device and control method of the presentinvention. Included in the system is a thermostat 20, a centralcontroller (unnumbered), a condenser fan 31, a compressor 32, an indoorblower fan 33, a condenser 34, an evaporator 35, a throttling valve 36,a closed circuit refrigerant line 37, and an evaporator drain pan 38.

The thermostat 20 includes a user temperature setting device 21 andtemperature sensor 22, a user humidity setting device 23 and humiditysensor 24. Thermostat 20 further includes a user selectable occupyswitch 25 and a microprocessor 26. The user selects the preferredtemperature and humidity settings on devices 21 and 23 respectfully.These settings are converted to a digital signal and communicated to themicroprocessor 26. In one embodiment these devices are digitaltouchpads. The user also selects the occupy mode from the switch 25corresponding to whether the enclosed space will be occupied byindividuals or not and the corresponding signal is communicated to themicroprocessor 26. The temperature sensor 22 and the humidity sensor 24generate a digital signal which corresponds to the temperature andhumidity in the enclosed space. These signals are also communicated tothe microprocessor 26.

The microprocessor compares the signals from the temperature sensor withthe corresponding user setting, and the signals from the humidity sensorwith the corresponding user setting and generates error signalscorresponding to their differences. The microprocessor uses the errorsignals together with the occupy mode setting and an algorithm based onthe limits described above to generate low voltage signals correspondingto indoor blower fan commands and compressor commands. These low voltagesignals are communicated to the HVAC system directly from thethermostat. This is a significant difference over the prior art. Thethermostat of the present invention is self contained and is capable ofdirectly controlling single speed and variable speed HVAC systems. Forinstance U.S. Pat. No. 5,062,276 teaches the use of a separate systemcontroller for controlling variable speed systems. The system describedin the '276 patent requires thermostat and humidistat separate and apartfrom the system controller and cannot be applied to single speedsystems.

During the cyclic operation of an air conditioner or heat pump operatingin a high humidity condition, the indoor blower operation should bediscontinued while the compressor is off to avoid re-evaporation ofmoisture which remains in the drain pan 38 and on the coil surface ofthe evaporator 35. This re-evaporation occurs to such an extent as tonullify the moisture removal of a typical system at some operatingconditions. The thermostat of the present invention prohibits indoorblower operation for 5 minutes immediately following a compressor shutdown while the humidity is above the humidity setpoint. This will limitre-evaporation by allowing the accumulated moisture on the evaporatorcoil and in the drain pan to drain off. One embodiment of the presentinvention incorporates a fan switch which allows for continuousoperation of the indoor blower fan independent of the rest of the HVACsystem subject to the limits described above.

Referring to FIG. 4 there is illustrated a functional flow diagramcorresponding to the algorithm logic of the microprocessor of oneembodiment of the present invention as described herein above for theoccupied setting. The logic of the present invention develops fourdifferent operational responses 41, 42, 43, and 44 determined fromdifferent temperature and humidity settings, sensed temperature andhumidity conditions, and occupancy status. With reference to FIG. 1,region I corresponds to operation response "Normal" mode 41, region IIto operation response "Cool & Dehum" mode 42, region III to HVAC SystemOff mode 43, and region IV to "Cool to Dehum" mode 44. The temperatureof the room T is first determined and then compared to the temperaturesetpoint T_(sp). If the temperature of the room is more than 3° F.colder than the temperature setpoint then operational response 43 iscommanded and the HVAC system is turned off. The humidity is thendetermined and combined with the temperature data. For instance,referring to FIG. 4, if the temperature of the room T is colder than, orequal to, the temperature setpoint T_(sp), but is no cooler than 3° lessthan the set point, then the operational mode will be determined basedon humidity. If the sensed humidity in the room RH is lower than thehumidity setpoint H_(sp) then again the operational response 43 will becommanded and the HVAC system will be turned off. If the sensed humidityis more than 6% higher than the humidity setpoint then operational mode44 will be commanded and the HVAC system will be commanded to remove thelatent heat within the room by either cycling or slowing the indoorblower and reducing the duty cycle of the compressor.

The sloped portion of region IV illustrates the condition where thesensed humidity in the room is greater than the humidity setpoint but isless than 6% higher than the humidity setpoint. In this situation theroom is not humid enough to necessitate "overcooling" the space. If thesensed humidity in the room is more than 2% greater than the humiditysetpoint for every 1° F. of difference between the sensed roomtemperature and the temperature setpoint then operational response 44will again be commanded. In the case where the sensed temperature of theroom is warmer than the temperature setpoint some level of sensiblecooling is required. If the sensed humidity in the room is lower thanthe humidity setpoint then operational mode 41 is commanded and the HVACsystem is operated in a "Normal" mode to reduce the sensible heat. Ifthe sensed humidity in the room is greater than the humidity setpointthen operational mode 42 is commanded and the indoor blower is cycled onand off or slowed down to improve the latent heat removal of the system.The same functional flow diagram holds for the unoccupied setting forthe system subject to the rules described herein above.

What is claimed is:
 1. A control device for controlling a climatecontrol system of an enclosed space, the climate control system having afan and a compressor, the control device comprising:a humidity sensorcapable of perceiving the humidity in the enclosed space and providing asignal corresponding to the perceived humidity; a temperature sensorcapable of perceiving the temperature in the enclosed space andproviding a signal corresponding to the perceived temperature; amicroprocessor operatively connected to receive the signals from thehumidity sensor and the temperature sensor for comparing the perceivedtemperature and humidity to a predetermined temperature and apredetermined humidity setting, and for sending separate enablingsignals to the fan and the compressor; the microprocessor operative in afirst operational mode to send a separate signal to the fan and to thecompressor enabling the fan and compressor to operate at full capacity,and the microprocessor operative in a second operational mode to send asignal enabling the compressor to operate at full capacity and to send aseparate signal enabling the fan to operate at less than full capacity.2. A control device according to claim 1 wherein the microprocessor isfurther operative in a third operational mode to send a signal enablingthe compressor to operate at less than full capacity and to send aseparate signal enabling the fan to operate at less than full capacity.3. A control device according to claim 1 wherein the microprocessor isfurther operative in a fourth operational mode to prohibit the operationof the fan for a predetermined time interval.
 4. A method of controllinga climate control system of an enclosed space, the climate controlsystem having an evaporator, a fan and a compressor, the methodcomprising:sensing the humidity in the enclosed space; sensing thetemperature in the enclosed space; comparing the sensed temperature andthe sensed humidity with a desired temperature and a desired humidity;and controlling the climate control system to cause the temperature andhumidity of the enclosed space to come within predetermined limits;wherein the controlling step includes the step of prohibiting of theoperation of the fan for a predetermined time interval following acompressor shutdown to allow accumulated moisture on the evaporator todrain off.
 5. A method according to claim 4 wherein the controlling stepfurther includes the steps of:setting the desired temperature; settingthe predetermined humidity; and choosing an occupancy setting forselecting the predetermined temperature and humidity limits.
 6. A systemfor conditioning the climate of an enclosed space, the systemcomprising:a fan; a compressor; a humidity sensor capable of perceivingthe humidity in the enclosed space and providing a signal correspondingto the perceived humidity; a temperature sensor capable of perceivingthe temperature in the enclosed space and providing a signalcorresponding to the perceived temperature; a microprocessor operativelyconnected to receive the signals from the humidity sensor and thetemperature sensor for comparing the perceived temperature and humidityto a predetermined temperature and a predetermined humidity setting, andfor sending separate enabling signals to the fan and the compressor; themicroprocessor operative in a first operational mode to send a separatesignal to the fan and to the compressor enabling the fan and compressorto operate at full capacity, and the microprocessor operative in asecond operational mode to send a signal enabling the compressor tooperate at full capacity and to send a separate signal enabling the fanto operate at less than full capacity.
 7. A climate control systemaccording to claim 6 wherein the microprocessor is further operative ina third operational mode to send a signal enabling the compressor tooperate at less than full capacity and to send a separate signalenabling the fan to operate at less than full capacity.
 8. A climatecontrol system according to claim 6 wherein the microprocessor isfurther operative in a fourth operational mode to prohibit the operationof the fan for a predetermined time interval.